@glyn.hudson or @TrystanLea are the only people I think who are qualified to comment here about iotawatt.
You can of course connect the emonTx to an emonBase using a wired connection. The Pi’s serial connection is already in use in the emonPi, and it’s that which means that radio, or something like a Pi Zero W or ESP8266 sending to the emonPi via Wi-Fi and your router, or a programmer sending via USB, are the only options available to you.
Just checking that I understand, but the Pi serial connection is being used for the RJ45 port (the one I can connect a temperature sensor to)?
No. The RJ45 I/O port on the “Shield”, or the measurement side, carries the One-wire bus for the temperature sensors and the pulse inputs. It’s the serial output of the measurement side (equivalent to the programmer port of your emonTx when you monitor the output via the IDE) that connects to the serial port of the RPi on pins 8 & 10 of the GPIO connector to physically transfer the data into the Pi.